1. Field of the Invention
This invention relates to the field of electrolytic fuel cells and more particularly to two fuel cell hybrid systems.
2. Background of the Invention
Having a clean reliable source of power, close at hand, is foremost on the minds of many building engineers, municipal managers, automotive designers and government energy policy makers. Indeed, the recently proposed National Energy Security Act of 2001 establishes a 10 percent investment credit for purchase of distributed power.
Fuel cells hold promise as such a source of power, particularly in light of their xe2x80x9csix ninesxe2x80x9d reliability, wherein they are capable of delivering uninterrupted power 99.9999 percent of the time. Aside from reliability, fuel cells also offer efficiency advantages, inasmuch as fuel cells convert chemical energy directly to electricity. As such, the Carnot cycle limitations seen typical heat-to-work conversion systems do not exist in fuel cell energy production scenarios.
Another advantage of fuel cells is that the cells themselves have no moving parts. Rather, the xe2x80x9cbalance of plantxe2x80x9d such as pumps, blowers, heat exchangers and other systems, are more likely to fail first. Current fuel cell applications suffer from unreliabilities associated with the balance of plant scenario. All fuel cells, especially high temperature fuel cells, require spent fuel/waste heat recovery subsystems. High temperature fuel cells typically are targeted for scenarios requiring more than one megawatt of power output.
Low temperature fuel cells, for example those utilizing proton exchange membranes, are typically targeted for those applications requiring less than one megawatt of power. These low temperature cells require fuel reforming subsystems to process such feed stocks as natural gas, propane or other readily stored fuels into carbon dioxide and hydrogen. Such fuel reforming subsystems (actually compact petrochemical plants), involve direct combustion of some of the fuel for making steam and to supply reforming energy. As such, fuel reforming lowers the efficiency of the entire system. By contrast, the aforementioned high temperature fuel cells such as the molten carbonate fuel cell (MCFC) and the solid oxide fuel cell (SOFC) can process natural gas or other hydrocarbon fuels into suitable fuels (carbon dioxide and hydrogen) directly, i.e., with the waste heat of the fuel cell.
Solid-state oxide fuel cells (SOFC""s) are particularly durable and economic. Solid electrolyte fuel cells exhibit a much lower rate of corrosion reactions and of electrolyte loss than liquid electrolytic fuel cells. Also, fuel processing for SOFC""s is less expensive than for liquid electrolytic fuel cells.
SOFC""s typically operate at a temperature of approximately 900xc2x0 C. This high temperature is required in order to obtain adequate electrolyte conductivity. As noted supra, operation at so high a temperature requires providing means for disposing of the heat released by the cell. Usually, the fuel cell is integrated with a heat engine, forming xe2x80x9cfuel cell/heat engine hybrids.xe2x80x9d These hybrids use excess heat from the fuel cells to generate steam which in turn is utilized to drive small micro-turbines for additional electricity generation. U.S. Pat. No. 5,541,014 discusses how gas turbines are essential systems components of these hybrid applications. However, these hybrids detract from the advantages of fuel cells in that they are bulky and noisy, they produce vibrations, and they comprise a multitude of moving parts that require frequent maintenance. As such, the inherent advantages of fuel cells, such as modularity, scalability, quietness, and low maintenance, are replaced with rotating equipment issues.
U.S. Pat. No. 5,541,014 to Micheli et al, utilizes an SOFC/MCFC in an effort to capture CO liberated during syngas production in the SOFC phase. However, both fuel cells are high temperature fuel cells. Indeed, MCFC operates at approximately 650xc2x0 C. Also, inasmuch as the electrolyte in MCFC is liquid, corrosion and electrolyte loss are possible.
A need exists in the art for a fuel cell hybrid system with a balance of plant approaching the six nines reliability of the cells themselves. The system would combine the advantages of both high temperature fuel cells and low temperature fuel cells, but without the disadvantages of each system. Also, the system would be modular and scalable so as to be attractive for use in the transportation and propulsion power sectors. And, the system should be adaptable to hydrogen co-generation in industrial settings.
It is an object of the present invention to provide a hybrid fuel cell system that overcomes many of the disadvantages of the prior art.
It is another object of the present invention to provide a fully integrated hybrid fuel cell system. A feature of the present invention is the use of two fuel cells in tandem whereby a solid oxide fuel cell is utilized to selectively introduce oxygen, electrochemically into a fuel stream to supercharge the fuel stream with oxygen for more efficient thermodynamic conversion by a low temperature fuel cell. An advantage of the present invention is full compatibility between the system""s components.
It is yet another object of the present invention to provide a hybrid fuel cell that allows efficient integration of the heat released by the system. A feature of the present invention is the use of two fuel cells in an electric power device where the heat released by a high temperature cell is utilized by a low temperature cell. An advantage of the present invention is that it efficiently utilizes heat released by the high temperature fuel cell, so as to raise efficiency of the hybrid system by 15-25 percent over that typically seen by each fuel cell operating independently.
It is yet another object of the present invention to provide a hybrid fuel cell system that employs both a high temperature fuel cell and a low temperature fuel cell. A feature of the invention is that the high temperature fuel cell (having an anionic electrolyte) only partially consumes, yet completely reforms incoming feedstock so that the more-efficient low temperature fuel cell (having a cationic electrolyte) can completely oxidize the remaining, reformed feedstock. An advantage of the invention is that efficiencies of between 65 percent and 75 percent are realized.
Briefly, the invention provides for a device for converting chemical energy to electricity, the device comprising a high temperature fuel cell having a means for partially oxidizing, and completely reforming fuel, and a low temperature fuel cell juxtaposed to said high temperature fuel cell so as to utilize the remaining, reformed fuel from the high temperature fuel cell.
Also provided is a method for producing electricity comprising directing fuel to a first fuel cell, completely oxidizing a first portion of the fuel and partially oxidizing a second portion of the fuel, directing the second fuel portion to a second fuel cell, allowing the first fuel cell to utilize the first portion of the fuel to produce electricity; and allowing the second fuel cell to utilize the second portion of the fuel to produce electricity.